1. Field of the Invention
(Technical Field)
The presently claimed invention relates to optics and more particularly to two mirror unobscured telescope designs of compact “Schiefspiegler”, off axis Cassegrain geometry, incorporating aspheres, tilted and decentered secondary, and tilted focal surfaces, which serve as fast, high resolution, moderately wide field telescopes/collimators. The tilted focal surfaces allow for new unique and desirable properties of the unobscured systems.
2. Background Art
Since the popularization of affordable amateur telescopes, there has been an ongoing argument between the reflector owners and the refractor owners, over which system is better. Reflecting telescopes have no color aberrations, but the central obscuration from the secondary causes loss of light as well as diffraction effects which degrade the image. The central obscuration in the pupil causes a reduction of the Strehl ratio of a point image and a decrease in mid spatial frequencies in the Optical Modulation Transfer Function (MTF). In addition, the secondary mirror support, or “spider”, gives rise to the familiar “star” shaped diffraction pattern about bright stars. Refracting telescopes have no central obscurations but suffer from color aberrations, which can usually be only partially corrected using two lens elements. These color effects are not present in reflecting telescopes. Further, two element refractor apertures are smaller than reflectors for the same price, affording less light gathering power, or with more lens elements get much more expensive, heavier, and start having problems with unwanted reflections and stray light.
More complicated optical systems, such as the many variations of Schmidt-Cassegrains with refractive correctors, can, like refractors, only be optimized in a particular region of the spectrum and often suffer from compromises made in surface figure complexity to lessen the cost of the optical system.
Unobscured all reflecting telescopes designs suffer from neither MTF degradation nor color aberrations. Still, very few unobscured system designs have been produced in any quantity due to other issues. The designs in the literature are typically of small aperture, high f/number, and are often complicated in alignment and mounting.
The problem with prior art centered two mirror visual optical systems is the secondary and spider support:
occult light—reducing system throughput;
produce diffraction effects, which causes the focus of the system to be broader, thus lowering the MTF of the system; and
produce opto-mechanical mounting problems due to the conflicting requirements of simultaneously mounting the secondary mirror to tolerances while minimizing the shadow cast by the secondary holder and spider.
The problems with prior art one and two mirror eccentric pupil or off axis designs for visual use are:
they have troubling focal plane tilt;
they are hard to baffle properly, and
they have smaller usable fields of view than their equivalent diameter centered optical systems.
For collimator use, prior art designs are centered systems which have obscuring optics which:
occult light and thereby underfill the optical system they are used in conjunction with;
reflections from the target retical produce reflections which can cause narcissus effects in the system under test; and
the reflecting area of the target retical can produce background signals of unknown and or uncontrolled amplitude.
For collimator use current eccentric pupil systems are limited in usable field of view and may have reflections from the target retical which are uncontrolled, possibly resulting in narcissus effects or providing a nominal field background which is radiometrically uncontrolled.
The problems with three and more mirror unobscured optical systems are their complexity and difficulty of alignment.
There are currently four prior art systems that attempt to provide a solution to the existing problems. These include the Schiefspiegler telescope, an unobscured, tilted field Newtonian telescope, a notionally, eccentric pupil Cassegrain telescope or Ritchey-Chre´tien, (R-C), designs. The shortcomings of these devices are many. The Schiefspeigler telescope is too “slow” due to a high f/number and low throughput and too small a working field of view. The unobscured Newtonian device is also similarly “slow”. The eccentric pupil Cassegrain (or R-C) tilted field of view and non-gut ray centered focus “walk” or focus remains on the geometric axis of symmetry while shifting from the gut ray. The three or more mirror systems are more costly to build and difficult to align.
Conventional wisdom in optical system design is to avoid tilts and decenters, except on an “individually compensating basis” so as to allow the entire system to be simply modeled using classical aberration theory. Modeling this system using classical aberration theory would be just so “messy” that optical designers have avoided doing it—hence we have not seen this design before. An expert, who is also an amateur astronomer, who has looked into midsized, reasonably fast, unobscured designs for amateur telescope, has proposed that this would require 3 or more mirror elements.
Quoting from the conclusion of a recent review article, “The World of Unobstructed Reflecting Telescopes” by José Sasián: “The known designs cover very well the span of small (3 to 5 inches) and medium apertures (6 to 8 inches) with great practicality and transportability. However, for larger apertures (10 inches or more), there is a need for very compact and moderately fast designs, (f/8 to f/15). These designs will probably require three mirrors and a double-curvature surface like the large Tri-Schiefspiegler discussed in section . . . . ”
The prior art approaches (other than the Schiefspeigler, where it hardly matters due to the very high f/number) do not even attempt to control the tilt of the focal plane of the system. For visual use the tilted focal plane, when brought to focus in the eye, causes the image at one side to be out of focus in one direction, while at the other side, it is out of focus in the other direction. Additionally, baffling against stray light becomes a serious issue for low f/number Cassegrain systems. The incidence angle of the gut ray to the focal plane surface causes problems for the prior art system when used visually. As the f/numbers are reduced, or as power and field of view of an eyepiece increase, the focal surface of the image in the eye diverges from the retina. FIG. 1 shows the large angle, (9.5 degrees), which the focal surface makes with respect to the gut ray of a nominally 10″ aperture, f/7 eccentric pupil Cassegrain which can also be baffled. If the system is compressed radially, to lessen this angle to the focal surface, the baffling will fail. Thus, for low f/number systems, the off axis Cassegrain has a necessarily large angle at the focal surface if the stray light is suppressed.
The nine 9.5 degree incidence angle of the gut ray to the focal plane surface causes problems for the system when used visually. As the f/numbers are reduced, or as power and field of view of an eyepiece increase, the focal surface of the image in the eye diverges from the retina. FIG. 2 shows an “ideal” paraxial optical model of a f/7 Off Axis Cassegrain with 9.6 degree focal surface tilt. Assuming a 1″ efl eyepiece and a 9.6 degree image tilt in eye at 70×1″ efl eye, (70×), the image tilt on the back of the eye is 9.6 degrees. This situation worsens with increase in power. With a ½″ efl eyepiece, (140×), as shown in FIG. 3, things are a factor of 2 worse, with the image tilt on the back of the eye now 19.2 degrees.
A modest image tilt is not a particularly vexing problem for imaging systems with film or a CCD, (Charge Coupled Device), electronic imaging array at the focal lane. Linear distortion is a small problem. On the other hand, visual imaging with an eyepiece may be a serious problem, especially at system f/numbers of f/8 or less. This would also be a problem with the off axis Newtonian Telescopes at lower f/numbers although the baffling requirement becomes more severe for the Cassegrain.
Control of the tilt of the focal plane allows the system to be used for lower f/number, a wider field of view, higher resolution and better MTF. For visual and imaging optics this allows much better focusing properties and the ability to aperture the unobscured pupil with an iris. For collimating and IR scene generating optics, the tilt of the focal plane allows reflections from the target reticle to be controlled eliminating Narcissus and allowing the field over the reflective portion of the retical to be illuminated with a constant irradiance.